The overall goal of this project is to understand the mechanism of DNA replication through difficult-to-replicate sites of DNA. These would include sites predicted to form DNA secondary structure such as telomeres and fragile sites. Telomeres consist of repetitive, G-C rich DNA and have long been considered natural impediments to the replication fork machinery. Fragile sites are expressed as gaps or breaks in chromosomal DNA that occur during conditions of replication stress. Telomeres and fragile sites are known to cause genomic instability and can lead to carcinogenesis when not properly maintained. CTC1-STN1-TEN1 (CST) is a novel protein complex, which was recently discovered in plants and vertebrates. STN1 and TEN1 are homologues to yeast proteins, which are involved in telomere maintenance and end-protection. In humans, CST was shown to localize to telomeres. Depletion of human CST subunits results in hallmarks of genomic instability, i.e. increased 3H2AX foci and chromatin bridges, as well as telomere abnormalities such as increased G-overhangs and telomere signal loss. Results from several labs suggest that CST has both telomeric and non- telomeric functions. Importantly, CST was shown to interact with DNA polymerase 1-primase (pol 1), which initiates DNA synthesis. Preliminary results presented in this proposal suggest that CST functions in telomere replication and replication fork reinitiation. We propose that CST recruits DNA pol 1 to reinitiate DNA synthesis at sites of dificult-to-replicate DNA, which may stall DNA replication. In aim 1, the role of CST in telomere replication will be investigated. Studies will then be extended to determine whether CST is required to maintain chromosomal fragile sites. Finally, the extent to which interactions between CST and the RecQ helicases, WRN and BLM, are required to maintain telomere and fragile site stability will be measured. In aim 2, the mechanism by which CST promotes replication fork progression will be studied. Fork reinitiation after fork stalling and the levels of single-stranded DNA will be measured in CST-depleted cell lines. CST interactions with replication fork components and fork reinitiation factors will then be explored. Together, these studies will provide an understanding of how CST helps maintain genomic stability through difficult-to-replicate DNA.